1. Field of the Invention
This invention relates to the testing of underground formations or reservoirs. More particularly, this invention relates to a method and apparatus for isolating a downhole test tool from vibration and noise due to heave and/or drilling fluid circulation during formation testing.
2. Description of the Related Art
While drilling a well for commercial development of hydrocarbon reserves, several subterranean reservoirs and formations are encountered. In order to discover information about the formations, such as whether the reservoirs contain hydrocarbons, logging devices have been incorporated into drill strings to evaluate several characteristics of these reservoirs. Measurement-while-drilling systems (hereinafter MWD) have been developed that contain resistivity, nuclear and other logging devices which can constantly monitor formation and reservoir characteristics during drilling of well boreholes. The MWD systems can generate data that include information about the presence of hydrocarbon, saturation levels, and formation porosity. Telemetry systems have been developed for use with the MWD systems to transmit the data to the surface. A common telemetry method uses a mud-pulsed system, an example of which is found in U.S. Pat. No. 4,733,233 incorporated herein by reference. MWD systems provide real time analysis of the subterranean reservoirs.
Commercial development of hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible in order to evaluate the reservoir for commercial viability. Despite the advances in data acquisition during drilling using the MWD systems, it is often necessary to conduct further testing of the hydrocarbon reservoirs in order to obtain additional data. Therefore, after the well has been drilled, the hydrocarbon zones are often tested by other test equipment.
One type of post-drilling test involves producing fluid from the reservoir, collecting samples, shutting-in the well and allowing the pressure to build-up to a static level. This sequence may be repeated several times for different reservoirs within a given borehole. This type of test is known as a “Pressure Build-up Test.” One of the important aspects of the data collected during such a test is the pressure build-up information gathered after drawing the pressure down. From this data, information can be derived as to permeability and size of the reservoir. Further, actual samples of the reservoir fluid are obtained and tested to gather Pressure-Volume-Temperature data relevant to hydrocarbon distribution in the reservoir.
The drill string is often retrieved from the well borehole to perform these tests in an operation known as tripping. A different tool designed for the testing is then run into the well borehole. A wireline is then used to lower a test tool into the well borehole. The test tool sometimes utilizes packers for isolating the reservoir. Alternatively, a wire line can be lowered from the surface, into a landing receptacle located within a drill string test tool, establishing electrical signal communication between the surface and the test assembly. Regardless of the type of test tool and type of communication system used, the amount of time and money required for retrieving the drill string and/or running a second test tool into the borehole is significant. Further, if the borehole is highly deviated, a wire line tool is difficult to use to perform the testing.
Various MWD tools have been developed to allow for the pressure testing and fluid sampling of potential hydrocarbon reservoirs as soon as the borehole has been drilled into the reservoir, without removal of the drill string. These MWD tools also reduce the risks associated with pressure kick, because the drilling fluid pressure can be monitored and maintained better when tripping is avoided.
The typical MWD tool, however, suffers in that vibrations caused by flowing drilling fluid, mud pumps, drilling motors and surface equipment are transmitted to the test device through the drill string or even directly in the case of flowing drilling fluid. These vibrations often adversely affect test results, because the downhole instrumentation can be too sensitive to operate effectively in mechanically noisy environment.
Another problem is associated with vertical movement known as heave encountered when drilling in an offshore environment. Heave movement can cause pressure leaks where probe sealing pads and packers engage the borehole wall to form a seal. Heave movement can also result in excessive wear on soft materials used for sealing against the borehole wall. Although such heave is normally associated with offshore drilling, any unwanted vertical movement while a seal is engaged with the borehole wall can damage the seal material or cause unwanted leaks. Therefore, the use of the term heave is not meant to limit the usefulness of the present invention to offshore drilling environments. The present invention addresses the need to have a MWD tool that provides protection to sensitive test devices and protects soft sealing materials from unwanted movements that cause excessive wear on such materials.